Low molecular weight compression ignition fuel

ABSTRACT

An improved hydrocarbon-based compression ignition fuel boils in the naphtha boiling range and comprises hydrocarbons having from 5 to about 14 carbon atoms, preferably predominantly normal paraffins having chain lengths from 6 to about 12 carbons. The fuel has an average cetane number ranging from 40 to 80 and a Reid vapor pressure of at least 2 psig to ensure safety in handling and storage. Pentane and/or oxygenated hydrocarbons such as dimethoxymethane can be added to reduce emissions and provide sufficient vapor pressure for safe handling and storage.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This patent application is a continuation-in-part of U.S. patent application Ser. No. 09/347,030, filed on Jul. 2, 1999.

CONTRACTUAL ORIGIN OF THE INVENTION

[0002] The United States Government has rights in this invention under Contract No. DE-AC36-99GO-10337 between the U.S. Department of Energy and the National Renewable Energy Laboratory (NREL, operated for the U.S. Department of Energy by Midwest Research Institute).

FIELD OF INVENTION

[0003] The present invention pertains generally to fuels for compression ignition internal combustion engines, and particularly to naphtha-type fuels which offer high cetane quality and safe operations in storage and use.

BACKGROUND OF THE INVENTION

[0004] Conventional internal combustion fuels fall into two general categories: 1) a naphtha-type gasoline and 2) distillate type diesel fuel. The two general categories have been established for safety in handling and fuel storage. The naphtha-type fuel is considered safe because it has sufficient vapor pressure to provide a rich vapor mixture in the storage tank, which is depleted in oxygen content and not combustible. The distillate-type fuel is considered safe because it has a low vapor pressure which yields a lean vapor mixture in the storage tank which is not combustible due to a lack of fuel mixed with ambient oxygen. The petroleum industry has developed refining processes to improve the octane number of components in the naphtha boiling range to 85 or above required for spark ignition engines with compression ratios near 8 or above.

[0005] Distillate components generally have natural cetane quality of 40 or above required for compression ignition engines.

[0006] Standard Fischer-Tropsch diesel fuel is produced from synthesis gas (hydrogen and carbon monoxide) using one of many Fischer-Tropsch catalysts and processes to generate products primarily composed of straight-chain paraffins which are subsequently processed and distilled into a boiling range similar to petroleum diesel fuel. Standard Fischer-Tropsch diesel fuel has excellent ignition delay (cetane number) quality, but it also creates some performance problems in cold climates because of paraffin wax precipitation. The standard Fischer-Tropsch diesel fuel is known to reduce emissions of particulate matter and other toxic compounds. Standard Fischer-Tropsch diesel fuel may also have beneficial effects for reducing emissions of oxides of nitrogen. Fischer-Tropsch naphtha is generated as a by-product of the Fischer-Tropsch process.

[0007] Petroleum-derived diesel fuel is produced from crude oil that is distilled or processed and distilled into a distillate boiling range of about 185° C. to 345° C. Petroleum-derived gasoline is produced from crude oil which is highly processed through cracking, reforming, alkylation, isomerization, and other means to increase the aromatic content, olefin content, and chain branching of paraffins to the improve octane number in a final blend in the naphtha-boiling range (typically from 36° C. to 219° C.). Straight-chain paraffins present in the original crude oil are minimized as part of the processing scheme. Octane number is also enhanced in gasoline through the addition of certain oxygenated components demonstrated to have high-octane quality.

[0008] U.S. Pat. No. 5,611,912 (assigned to Mobil Oil Corp.) discloses a process for the production of diesel fuel with a high cetane number at low cloud point, by hydrocracking highly aromatic fractions obtained from catalytic cracking operations. The fraction of hydrocracker effluent boiling between 400-1000 deg. F. is catalytically dewaxed to obtain a cloud point of 41 deg. F. or less. This work is directed at processing crude oil feed streams that are rich in aromatic compounds to a product suitable for use as a diesel fuel by treatment over a hydrocracking catalysts. Unlike the present invention, it does not address the formulation of compression ignition engine fuels from light alkane hydrocarbons such as would be found in a Fischer-Tropsch synthesis product stream. This work also does not address the benefits derived from incorporation of alkoxy containing moieties in a diesel fuel formulated from straight chain paraffins as does the present invention.

[0009] U.S. Pat. No. 5,639,931 (assigned to Mobil Oil Corp.) discloses a process for converting mixtures of olefins and isoparaffins to diesel fuel blending components by contacting the mixture with a zeolite catalyst (MCM-22, -36, -49, or -56) to provide a product containing a diesel fuel.

[0010] U.S. Pat. No. 5,780,703 (assigned to Mobil Oil Corp.) discloses a process for converting mixtures of olefins and isoparaffins to diesel fuel blending components by contacting the mixture with a catalyst comprising an acidic solid comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal to provide a diesel fuel.

[0011] The above noted U.S. Pat. No. 5,639,931 (directed at the critical structure of a catalyst) and U.S. Pat. No. 5,780,703 (directed at the active metal incorporated into a catalyst) address the nature of catalysts suitable for converting a mixture containing a branched paraffin and an olefin of low carbon number into a product liquid in which the olefin is alkylated by the paraffin, and which is suitable for use as a diesel fuel. It does not address the production of diesel fuels from straight chain paraffins nor does it address the improvements in the contents of combustion gasses obtained by the addition of compounds containing alkoxy or methoxy moieties to such diesel fuels, as does the present invention.

[0012] U.S. Pat. No. 5,763,716 (assigned to Rentech, Inc.) discloses a process of converting a feed of hydrocarbon-containing gases into liquid hydrocarbon products including a first reaction of converting the feed into 1-2.5 parts hydrogen to 1 part carbon monoxide in the presence of carbon dioxide, then reacting the hydrogen and carbon monoxide in a Fischer-Tropsch synthesis reactor using a promoted iron oxide catalyst slurry to form liquid hydrocarbon products. The carbon dioxide from the first and second reactions is separated from the product streams, and at least a portion is recycled into the first reaction feed. The hydrocarbon products are separated by distillation, and a normally gaseous portion of the separated products are further reacted in another Fischer-Tropsch synthesis reactor to form additional liquid hydrocarbon products. This work is directed at maximizing the yield of high boiling hydrocarbons produced in a Fischer-Tropsch reaction, but does not specifically address the advantages disclosed in the present invention obtained by formulating compression ignition engine fuel from primarily straight chain paraffins. It also does not address the advantages of incorporating oxygen containing moieties into such a fuel.

[0013] U.S. Pat. Nos. 5,766,274 and 5,689,031 (both assigned to Exxon) disclose two processes for producing clean distillates useful as jet fuels or blending stocks or as diesel fuel or blending stocks, respectively. Fischer-Tropsch waxes are separated into heavier and lighter fractions, then the lighter fraction is separated and the heavier fraction is hydroisomerized with the portions of the light fraction above about 475° F. or below about 500° F., respectively. The isomerized products are blended with the untreated portion of the lighter fraction to produce jet fuels or diesel fuels, respectively. The work in these two patents is directed toward the conversion of high boiling compounds derived from Fischer-Tropsch synthesis into a product which when blended with lower boiling products of Fischer-Tropsch synthesis is suitable for use as jet and diesel fuel. This work does not address blends containing exclusively low molecular weight straight chain paraffins as diesel fuel.

[0014] U.S. Pat. No. 5,752,989 (assigned to Ethyl Corp.) discloses a diesel fuel additive comprising a mixture of a dispersant and a carrier. The dispersant comprises at least one of polyalkylene succinimides and polyalkylene amines. The carrier comprises at least one oxygenate selected from polyalkoxylated ethers, polyalkoxylated phenols, polyox-alkylated esters and polyoxalkylated amines. This work is directed specifically at one benefit of adding the subject compounds to a conventional diesel fuel, namely that the injectors of a compression ignition engine are subject to lower amounts of carbon deposit during operation. This work does not address the advantages in lower hydrocarbon emissions in combustion gases achieved by incorporation of an oxygen containing moiety as a significant component in diesel fuel, as is done in the present work, nor does it address the use of a mixture of straight chain paraffins as a diesel fuel.

[0015] U.S. Pat. No. 5,746,783 (assigned to Martin Marietta Energy Systems) discloses methods and compositions for controlling nitrogen oxides emissions from diesel engines. Small amounts of urea or a triazine compound (methylol melamines) are added to the diesel fuel. These materials, generally insoluble in diesel fuel, are suspended therein as microemulsions. This work is not directed at the use of straight chain paraffins as a diesel fuel. While it does address the reduction of certain types of emissions in combustion gasses (oxides of nitrogen) through the inclusion of nitrogen containing compounds in diesel fuel, it does not address reduction of hydrocarbon emissions through the inclusion of an oxygen containing moiety in the fuel. Additionally, the compound taught in this work is insoluble in diesel fuel, necessitating the use of a complex microemulsion to incorporate the additive into the fuel, unlike the present invention which teaches the use of soluble additives to achieve improved emissions.

[0016] U.S. Pat. No. 5,746,785 (assigned to Southwest Research Inst.) discloses mixtures of alkoxy-terminated poly-oxymethylenes useful as diesel fuel additives. The resulting fuels have improved lubricity and reduced smoke formation, without degradation of the cetane number of the base diesel fuel. This work is directed at improving the combustion properties of conventionally formulated diesel fuels by the addition of an oxygen containing polymer having multiple ketonic functionality incorporated into its structure. This work was not directed at the formulation of liquids suitable for use as diesel fuel from straight chain paraffins.

[0017] U.S. Pat. No. 5,807,413 (assigned to Exxon) discloses producing a diesel engine fuel through the separation of a light density fraction from Fischer-Tropsch wax. This work is directed toward C₅-C₁₅ hydrocarbons having at least 80 wt % n-paraffins and limiting the alcohol content of the oxygen, the olefins, the aromatics, and the sulfur and the nitrogen content. This work does not identify a minimally acceptable cetane number, nor does it exhibit a sufficient Reid vapor pressure to ensure a safe rich mixture in storage as does applicants' claimed invention.

[0018] U.S. Pat. No. 6,056,793 (assigned to University of Kansas Center for Research) discloses a mixture of two components that create a compression ignition fuel. The two components include a light synthetic crude and a blendstock having an average molecular weight less than that of the light synthetic crude. While this work spans a very broad range from the heavy end of naphtha up to heavy distillate fuels, it fails to provide a vapor pressure threshold as does applicants' claimed invention, and it also does not disclose an overall average molecular weight within the applicants' range.

[0019] Jensen et al. describe in “Studies on Iron-Manganese Carbon Monoxide Catalysts”, Journal of Catalysis Vol. 92, pp. 98-108 (1985) certain bulk iron catalysts promoted with manganese oxide which enhance the formation of low-molecular weight olefins when used for carbon monoxide hydrogenation. This work was directed toward characterization of a catalyst which selectively produces low molecular weight, unsaturated hydrocarbons by Fischer-Tropsch synthesis. It does not address the production of diesel fuels from straight chain paraffins nor the advantages in combustion gas composition obtained by the incorporation of an oxygen containing moiety in such diesel fuels.

[0020] Maricq et al describe in “The Effect of Dimethoxy Methane Additive on Diesel Vehicle Particulate Emissions,” SAE Paper No. 982572 (San Francisco, October 1998) a 36±8 percent reduction in mass emissions of particulates with no change in NO_(x)emissions from a light-duty diesel engine when 16.6 percent of this additive is mixed with a base European diesel fuel. This work is directed at the improvement in combustion gas composition derived from adding an oxygen containing moiety to conventional diesel fuel and does not address formulation of diesel fuel from straight chain paraffins.

[0021] Applicant Bailey et al. present in “Cetane Number Prediction from Proton-Type Distribution and Relative Hydrogen Population,” SAE Technical Paper No. 861521, presented at International Fuels and Lubricants Meeting, Philadelphia, 1996, a theoretical model for predicting cetane numbers of primary reference fuels from parameters measurable by proton nuclear magnetic resonance. The technique is extended to include secondary reference fuels, pure hydrocarbons and commercial-type fuels.

[0022] Applicant Bailey's M.S. thesis, “Characterization of Alumina Supported Cobalt Copper Catalysts for Fischer-Tropsch Synthesis of Light Hydrocarbons,” on file at the Department of Mining and Fuels Engineering, University of Utah, describes the properties of such catalysts which are suitable for the production of C₂ to C₄ hydrocarbons.

SUMMARY OF THE INVENTION

[0023] The main aspect of the present invention is a compression ignition fuel based upon hydrocarbons in the naphtha boiling range.

[0024] Another aspect of the present invention is a compression ignition fuel which offers a relatively high average cetane number and minimizes harmful emissions when used in compression ignition engines.

[0025] Still another aspect of the present invention is a compression ignition fuel which offers excellent fuel economy combined with minimum harmful engine emissions.

[0026] Other aspects of this invention will appear from the following description and appended claims.

[0027] This invention takes the novel approach of formulating a safe naphtha-type fuel with high cetane quality for compression ignition. Conventional petroleum-derived diesel fuel consists of aromatic and paraffinic hydrocarbons which typically boil from about 185° C. to about 345° C. Certain products of diesel exhaust including particulates and some aromatic products are considered toxic. Controlling the chemical composition of diesel, or compression ignition fuel, to eliminate aromatic compounds and reducing the boiling-point range or molecular weight will yield significant reductions in mass emissions of toxic components.

[0028] Improving the efficiency of the engine will reduce the emissions of carbon dioxide, considered a greenhouse gas, which contributes to global warming trends. The compression ignition engine is approximately 30 percent more efficient than the spark ignition engine and may be selected for ultra-high efficiency engine systems required in the future for control of greenhouse gas emissions.

[0029] The present invention includes compositions of Fischer-Tropsch derived or petroleum crude oil-derived hydrocarbons and oxygenates comprising low molecular weight paraffinic hydrocarbons in the gasoline or naphtha boiling range (typically from 26° C. to 219° C.). The compositions would be primarily composed of straight-chain paraffins with an ideal composition consisting essentially of the normal paraffins hexane, heptane, octane, nonane, decane, undecane, and dodecane. The cetane numbers of these components are about 45, 56, 64, 72, 77, 88, and 90 respectively. The fuel compositions may include various combinations of single or individual compounds such as hexane, heptane, octane, nonane, decane, undecane, and dodecane. The fuel compositions may also contain varying percentage combinations of two or more of these compounds. Pentane, diethyl ether (DEE), dimethoxy-methane (DMM) or other high-vapor pressure oxygenates with high-cetane numbers may be added to provide sufficient vapor pressure in the final blend to yield a safe mixture with a Reid vapor pressure of at least 2 psig or greater. The structural formulas of these compounds are shown in Table 1 below.

[0030] In accordance with the invention, a hydrocarbon-based compression ignition fuel is provided which has an average boiling point in the naphtha boiling range (26° C. to 219° C.), comprising blends of at least two hydrocarbons having from 5 to about 14 carbon atoms per molecule, each yielding cetane numbers ranging from about 30 to about 90. Preferably, the hydrocarbons have chain lengths of from about 6 to about 12 carbon atoms, plus or minus one carbon atom, and are predominantly normal hydrocarbons. The hydrocarbons are also preferably predominantly saturated, or moderately unsaturated in structure. The blends should have TABLE 1 Com- Carbon pound Atoms Structural Formula Pentane 5 H₃C—CH₂—CH₂—CH₂—CH₃ Hexane 6 H₃C—CH₂—CH₂—CH₂—CH₂—CH₃ Heptane 7 H₃C—CH₂—CH₂—CH₂—CH₂—CH₂—CH₃ Octane 8 H₃C—CH₂—CH₂—CH₂—CH₂—CH₂—CH₂—CH₃ Nonane 9 H₃C (CH₂)₇ CH₃ Decane 10 H₃C (CH₂)₈ CH₃ Undecane 11 H₃C (CH₂)₉ CH₃ Dodecane 12 H₃C (CH₂)₁₀ CH₃ Diethyl- 4 H₃C—CH₂—O—CH₂—CH₃ ether Dimeth- 3 H₃C—O—CH₂—O—CH₃ oxy- methane

[0031] an average cetane number of at least 40, preferably in the range of from about 50 to about 60, plus or minus 3. Pentane and/or oxygenated hydrocarbons containing from 2 to about 4 carbon atoms and at least one oxygen atom can optionally be added to provide improved combustion and fuel emissions properties and to provide a Reid vapor pressure of at least 2 psig for safe handling and storage. Additionally, light straight-run naphtha (LSRN, also known as light petroleum naphtha and natural gasoline) having average cetane numbers in the range of 30 to 45 can be used as blending stock with the hydrocarbons described above to formulate fuel compositions meeting the desired standards.

[0032] The hydrocarbon blends of the invention can be produced through direct Fischer-Tropsch processing or by conversion of Fischer-Tropsch products to create the desired ranges of components. Alternatively, the fuel compositions can be produced through petroleum crude oil distillation processing followed by isolation or subsequent processing to create the desired ranges of components.

[0033] The fuel compositions of the invention can be used as fuels for conventional diesel engines, advanced compression ignition engines and even fuel cells. The fuel compositions claimed under this invention can be used in current technology diesel engines which operate on a compression ignition principle. The fuel compositions claimed can also be used in new generation direct injection compression ignition engines including advanced control systems designed to take advantage of the fuel composition to meet current and future emission standards, or other compression ignition engines without direct injection. The fuel compositions claimed can also be used in fuel cell engines, provided an onboard or stationary reformer is used to generate hydrogen for fuel cell applications. Such engine-fuel combinations can be used to power a variety of applications, including, but not limited to, highway or off-road vehicles, aircraft, watercraft or stationary power plants. When used to fuel such engines, the fuel compositions of the invention can produce excellent fuel economy while reducing emissions of particulate matter and other toxic products, nitrogen oxides, carbon monoxide and other regulated gaseous emissions.

[0034] Before explaining the disclosed embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of the particular compositions and applications described, since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] As outlined above, the fuel compositions of the present invention comprise blends of at least two hydrocarbons having from 5 to about 14 carbon atoms per molecule, each yielding cetane numbers ranging from 30 to about 90. Light straight-run naphtha (LSRN) can also be used as blending stock, and pentane or oxygenates can be added for particular purposes. The claimed fuel compositions encompass blends of the following components in the proportions indicated by volume percent in Table 2: TABLE 2 Component Volume Percent Pentane Zero to 15% Hexane Zero to 50% Heptane Zero to 50% Octane Zero to 50% Nonane Zero to 40% Decane Zero to 40% Undecane Zero to 40% Dodecane Zero to 40%

[0036] The components for these compositions can be obtained from any suitable and economic source, including Fischer-Tropsch syntheses and petroleum refining and distillation. Synthesis gas, a mixture of carbon monoxide and hydrogen, is obtainable from a variety of sources, including coal gasification and partial combustion of natural gas or biomass feedstocks, and can readily be subjected to Fischer-Tropsch synthesis reactions using suitable catalysts and process conditions to produce products containing suitable hydrocarbons for blending into compositions of the present invention. Oxygenates such as diethyl ether and dimethoxymethane are readily available from commercial sources. LSRN is obtained from crude oil distillation, and has low octane numbers, and therefore, reasonably adequate cetane numbers.

[0037] The fuel compositions claimed under this invention may be produced through a variety of chemical processes including separation processing in a conventional petroleum refinery used to isolate hexane, heptane, octane, nonane, decane, undecane, or dodecane, or any combination of these compounds. Processes used to create fuel blends within the scope of the invention may also include chemical reaction processes to thermally or catalytically crack higher molecular weight hydrocarbons into paraffins from about C₆ to C₁₂ carbon chain lengths, plus or minus one carbon atom. Other processes may include thermal or catalytic petroleum refinery processes which remove alkyl branches to generate straight-chain paraffins. Conventional distillation or adsorption separation processing may be used to isolate or concentrate the desired products. Conventional cracking and hydrotreating catalysts may also be used with appropriate modifications to process conditions to obtain the desired products.

[0038] The fuel compositions claimed under this invention may also be produced through catalytic Fischer-Tropsch processes, that directly generate low molecular weight normal paraffins ideally from about C₆ to C₂ carbon chain lengths, plus or minus one carbon atom, through control of process conditions, and/or through catalyst formulation. Catalyst formulations including binary or other combinations of active metals such as iron, cobalt, copper, manganese, nickel, or other active Fischer-Tropsch metals or chemicals as reaction modifiers may facilitate the production of the desired products. The fuel compositions claimed under this invention may also be produced through conventional Fischer Tropsch processes and catalysts which create longer chain hydrocarbons which are then thermally or catalytically cracked to yield products in the C₆ to C₁₂ carbon number-size range, plus or minus one carbon atom.

[0039] Since the fuel compositions of the present invention will normally have lower viscosity than conventional diesel fuels, lubricity additives are preferably included, in addition to other conventional fuel additives. Commercial additives for enhancing the lubricity of diesel fuels are readily available, and may improve the final performance of the fuel compositions of the present invention.

EXAMPLES

[0040] The invention will be further illustrated by the following non-limiting examples (fuel compositions are composed of the following ingredients in the volume percent proportions indicated): TABLE 3 Blend Blend Cetane RVP Compound No. Index A B C D E F G H I J LSRN 35 12.2 0 0 0 0 0 0 0 20 9 8 DMM 46 12.2 0 0 9 15 0 20 0 0 0 0 DEE 60 16 0 9 0 0 0 0 8 0 0 0 Pentane (C₅) 35 14.7 9 0 0 0 10 0 0 0 0 0 Hexane (C₆) 45 4.9 13 13 13 15 30 30 30 15 13 30 Heptane (C₇) 56 1.0 13 13 13 15 25 25 25 13 13 25 Octane (C₈) 64 0.1 13 13 13 15 20 20 20 12 13 20 Nonane (C₉) 72 0 13 13 13 10 10 5 12 10 13 12 Decane (C₁₀) 77 0 13 13 13 10 5 0 5 10 13 5 Undecane (C₁₁) 88 0 13 13 13 10 0 0 0 10 13 0 Dodecane (C₁₂) 90 0 13 13 13 10 0 0 0 10 13 0 Volume % 100 100 100 100 100 100 100 100 100 100 Avg. Cetane No. 67.1 69.4 68.0 64.2 54.9 52.9 57.6 63.4 67.1 55.6 Blend RVP Index 3.7 4.0 3.2 4.7 5.4 7.0 5.0 5.8 3.2 4.3 Blend RVP psi 2.8 3.0 2.5 3.4 3.8 4.8 3.6 4.1 2.5 3.2

[0041] The examples above show several blend combinations that produce calculated cetane numbers of 50 or better and have Reid vapor pressure ratings greater than at least 2 psig.

[0042] For the hypothetical examples illustrated in Table 3, the cetane numbers are estimated in a linear fashion as described by the following equation:

Blend Cetane Number=Σ(Volume Fraction Component)_(i) x(Cetane Number of Component)_(i) for i=1 to x, and x=the number of blend components  (1)

[0043] For example, Blend F, comprised of 5 components including: DMM at 20% by volume with a cetane number of 45, hexane at 30% by volume with a cetane number of 45, heptane at 25% by volume with a cetane number of 56, octane at 20% by volume with a cetane number of 64, and nonane at 5% by volume with a cetane number of 72 yields a Blend Cetane number equal to [(0.2)(45)+(0.3)(45)+(0.25)(56)+(0.2)(64)+(0.05)(72)], or 52.9. Linear, volume proportional weighting of cetane numbers of the blend components is widely accepted in the petroleum industry as a good method for estimating the cetane number of a final blend.

[0044] For the hypothetical examples illustrated in Table 3, the Reid vapor pressures (RVP) are estimated by using a vapor pressure-blending index, as shown by the following equation:

Blend RVP Index=Σ(Volume Fraction of Component)_(i) x(Reid Vapor Pressure)_(i) ¹²⁵ for i=1 to x, and x=the number of blend components  (2)

[0045] where the RVP of each blend component, listed in the first column of Table 3, is well known to persons skilled in the art. The LSRN blend component may be composed of different light hydrocarbons and thus its RVP value may vary. For this example, a RVP of 12.2 is assumed as a typical value. For example, Blend F, comprised of 5 components, including: DMM at 20% by volume with a RVP of 12.2, hexane at 30% by volume with a RVP of 4.9, heptane at 25% by volume with a RVP of 1.0, octane at 20% by volume with a RVP of 0.1, and nonane at 5% by volume with a RVP of 0.0 yields a Blend RVP equal to [(0.2)(12.2)^(1.25)+(0.3)(4.9)^(1.25)+(0.25)(1.0)^(1.25)+(0.2)( 0.1)^(1.25)], or 7.01. The Blend RVP Index is then converted to the Blend RVP by performing the inverse function, e.g., raising the Blend RVP Index to the 0.8 power. For example, Blend F would have a Blend RVP equal to its Blend RVP Index, or 7.01, raised to the 0.8 power which equals 4.75. The Blend RVP values for each of the ten formulated blends, A through J, are disclosed in the last row of Table 3.

[0046] Upon testing, the blended fuels are found to have average cetane numbers close to the values of 50-70 predicted by calculations and Reid vapor pressures of at least about 2 psig. The addition of about 5 to 20 percent of dimethoxymethane or diethyl ether is found to further reduce emissions when used as a compression ignition fuel without materially affecting engine operation.

Examples 1 and 2

[0047] As actual examples, a bench-scale blend and a 30 gallon blend containing 33 volume percent a hexane and 11 volume percent each of heptane, octane, nonane, decane, undecane, and dodecane were prepared and tested by an independent testing laboratory per ASTM D975-97. The results are shown in Table 4. Average blend RVP values were calculated independently for these two samples and were found to be 2.1 psi. TABLE 4 Test & Method Example 1 Example 2 Reid Vapor Pressure, psi @ 2.3 (average 1.9 (average 100° F. by mini Herzog of 5 runs) of 4 runs) Flash Point, ° F. <68 <75 ASTM D93-97 Water & Sediment, vol % <0.05 <0.05 ASTM D1796-83 Distillation, ° F. 390 390 ASTM D86-96 90% Volume Recovered Viscosity, cSt @ 40° C. 0.6918 0.7119 ASTM D445-94 Ash, weight % <0.001 <0.001 ASTM D482-95 Sulfur, weight% 0.003 0.004 ASTM D4294-98 Copper Corrosion 1B 1B 50° C. for 3 hours ASTM D130-94 Cetane # (calculated) 65.0 66.6 ASTM D4737-90 Cloud Point, ° C. -52 -51 ASTM D2500-91 Rarnsbottom Carbon <0.06 <0.06 Residue on 10% distillation residue per ASTM D524-95

[0048] Although the present invention has been described with reference to preferred embodiments, numerous modifications and variations can be made and still the result will come within the scope of the invention. No limitation with respect to the specific embodiments disclosed herein is intended or should be inferred. 

We claim: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A hydrocarbon-based compression ignition fuel composition in the naphtha boiling range comprising at least two hydrocarbon compounds containing from 5 to about 14 carbon atoms, said composition having a cetane number of at least 40, and said composition further having a Reid vapor pressure of at least 2 psig.
 2. The fuel composition of claim 1, wherein said composition has a cetane number preferably in the range of about 50 to about
 60. 3. The fuel composition of claim 1, wherein said hydrocarbon compounds comprise predominantly normal paraffins having chain lengths from about 6 to about 12 carbon atoms.
 4. The fuel composition of claim 1, wherein one of said hydrocarbon compounds is pentane.
 5. The fuel composition of claim 1, said composition further comprising an oxygenated hydrocarbon containing from 2 to about 4 carbon atoms and at least one oxygen atom, in a quantity effective to reduce engine emissions when said fuel is employed in a compression ignition engine.
 6. The fuel composition of claim 5, wherein said oxygenated hydrocarbon comprises dimethoxymethane.
 7. The fuel composition of claim 5, wherein said oxygenated hydrocarbon comprises diethyl ether.
 8. The fuel composition of claim 1, said composition further comprising light straight-run naphtha. 